1. Field of the Invention
The present invention relates to a sensing device which senses, by making an adsorption layer formed on an electrode provided on a piezoelectric piece adsorb a substance to be sensed in a sample fluid, the substance to be sensed based on a variation in natural frequency of the piezoelectric piece.
2. Description of the Related Art
As a device for sensing a trace substance in a solution or gas, there has been known a sensing device which uses QCM (Quarts Crystal Microbalance) formed by a quartz-crystal resonator which is a piezoelectric resonator mainly formed by an AT-cut quartz-crystal piece as a piezoelectric piece. A sensing device of this type senses a presence/absence or a concentration of a trace substance in a sample fluid by making the aforementioned quartz-crystal resonator that forms a quartz-crystal oscillator circuit adsorb the trace substance and detecting a difference between an oscillation frequency (resonance frequency) after the absorption of the trace substance and an oscillation frequency before the absorption of the trace substance or an oscillation frequency of a quartz-crystal resonator for reference in which the trace substance is not absorbed. Examples of the trace substance are dioxin which is an environmental pollutant in the air, a specific antigen in blood or serum, and so on, and the sensing device senses an extremely low concentration, for example, on ppb to ppt level, of these substances.
Such a sensing device is structured by forming an excitation electrode for oscillating the aforementioned piezoelectric piece on a surface of the piezoelectric piece, for instance, and by further stacking an absorption layer such as, for instance, an antibody that absorbs a trace substance on the excitation electrode. Further, by making the absorption layer absorb the trace substance as described above to measure an oscillation frequency of a quartz-crystal resonator, a presence/absence or a concentration of the trace substance in a sample fluid is calculated based on, for example, a previously determined calibration curve or threshold value.
Incidentally, a piezoelectric piece requires a period of time from the start of its oscillation to the stabilization of the oscillation, and when being oscillated in a liquid phase, it particularly requires a long period of time until the oscillation is stabilized. Further, also when the piezoelectric piece in a state of oscillating in a vapor phase in a stabilized manner is put in a liquid phase, it requires a long period of time for stabilizing the oscillation. Therefore, when a trace substance is sensed, there is a need to provide a waiting time for waiting until an oscillation frequency of the piezoelectric piece is stabilized to a predetermined value, before supplying a sample fluid to an absorption layer, for instance, and then to start the sensing (measurement) of a substance to be sensed after the waiting time is elapsed. However, it is extremely difficult to judge whether the oscillation frequency is stabilized or not, and experience and judgment of an operator, for instance, are necessary for detecting the trace substance in a short period of time with high accuracy. Specifically, if the measurement is started before the oscillation frequency of the piezoelectric piece is stabilized, the detection accuracy of the trace substance is lowered, and if the waiting time which is longer than necessary is provided, the period of time required for the measurement is increased. Further, when the aforementioned sample fluid is a liquid, a liquid such as a buffer solution, for instance, is supplied to the piezoelectric piece before the measurement to stabilize the oscillation frequency, but, since the longer period of time is required in the liquid, than in the gas, until the oscillation frequency of the piezoelectric piece is stabilized, it is further difficult to find out whether the oscillation frequency of the piezoelectric piece is stabilized or not.
Although Patent Documents 1 and 2 disclose a sensor and a system using a quartz-crystal resonator, no study has been made on the aforementioned problems.    [Patent Document 1] Japanese Patent Application Laid-open No. Hei 11-183479    [Patent Document 2] Translated National Publication of Patent Application No. 2005-530177